The present invention relates to a thermal protector suitably applied to a small battery pack used for a cellular phone and the like.
FIG. 16 shows one example of a conventional thermal protector incorporated in a small battery pack or the like. This thermal protector is configured so that a bimetal element 2 and a movable plate 3 are disposed in succession over a fixed conductive plate 1, the fixed conductive plate 1 being connected with a terminal 4, and the movable plate 3 being connected with a terminal 5.
In this thermal protector, when the ambient temperature of the bimetal element 2 rises to a predetermined temperature, the bimetal element 2 performs reversing operation. Thereby, a front end portion of the bimetal 2 pushes up the movable plate 3 via a protrusion 3a provided on the lower face of the movable plate 3. As a result, a movable contact 3b provided on the movable plate 3 is separated from a fixed contact 1a provided on the fixed conductive plate 1, by which the terminals 4 and 5 are made non-conductive electrically.
Therefore, the interposition of this thermal protector in a current-carrying path of the battery pack protects the battery from overcurrent or the like.
In the above-described thermal protector, the bimetal element 2 always applies a push-up force to the movable plate 3 because of the draw shape of the bimetal element 2, so that it is difficult to secure a pressing force of the movable contact 3b against the fixed contact 1a by means of a spring force of the movable plate 3.
In order to avoid this problem, it is necessary only that the size of the bimetal element 2 be restricted. However, if the size of the bimetal element 2 is restricted, a reversely driving force of the bimetal element 2 becomes insufficient, so that there is a fear that normal switching operation cannot be performed.
Specifically, for example, a state is considered in which after the contact 3b is raised by the reversing operation of the bimetal element 2 caused by a rise in ambient temperature, the ambient temperature lowers. In this case, a state sometimes occurs in which a force for maintaining the reversed state of the bimetal element 2 becomes lower than the spring force of the movable plate 3, by which the contact 3b is closed.
Also, in the thermal protector having a construction as described above, the movable plate 3 is often used by being bent into a chevron shape. In this case, since a case 6 has no allowance in the thickness direction, the movable plate 3 is incorporated in such a manner that the vertex of the movable plate 3 is pushed in the inside surface of the case 6. As a result, an actual span in which the bimetal element 2 raises the movable plate 3 is smaller than a span when the movable plate 3 is in a free state. That is to say, the upper limit of the above-described actual span is the height of the vertex of the movable plate 3 pushed in by the case 6.
As described above, if the sway span of the movable plate 3 is restricted, a stress applied to the bimetal element 2 increases to several times, so that a state occurs in which normal switching operation cannot be performed.
On the other hand, in the above-described conventional thermal protector, the protrusion 3a, which serves as a point of application for reversing operation, is provided on the lower face of the movable plate 3, which results in an increase in the thickness of the thermal protector.
In effect, despite the fact that the thermal protector used for a battery pack is required to be thin, it is difficult to decrease the total thickness of the fixed contact 1a and the movable contact 3b, and to secure a space for reversing operation of the bimetal element 2 in the combination with the movable plate 3 to perform switching operation without trouble by using a thin shape.
A thermal protector in which the bimetal element is disposed over the movable plate has been used practically. In the conventional thermal protector of this type, configuration is such that an end portion of the bimetal element is fitted lockingly to a locking claw projecting from the top face of the movable plate, which presents a problem in that the thickness of the whole of a switching mechanism section increases.
Also, a thermal protector using no movable plate, that is, a thermal protector that is configured so that the movable contact is installed to the bimetal element, and a load current is carried in that bimetal element has been used practically. In this thermal protector, it is difficult to install a thin contact to the bimetal element. Also, in the case where the contact is installed to the bimetal element, it is difficult to obtain stable quality because the characteristics of the bimetal element vary greatly.
The present invention has been made in view of the above situation, and accordingly an object thereof is to provide a thermal protector in which a thin shape can be realized without impaired stability of operation.
To achieve the above object, the present invention provides a thermal protector having a switching mechanism section comprising a fixed conductive plate which has a first terminal for connection to an outside circuit and is provided with a fixed contact on the upper face thereof; an elastic movable plate located over the fixed conductive plate, which has a second terminal for connection to an outside circuit at the rear end thereof and is provided with a movable contact opposed to the fixed contact on the lower face of a front end portion thereof; and a bimetal element located over the movable plate, which operates the movable plate by a warping force when a preset temperature is exceeded, whereby the movable contact is separated from the fixed contact, wherein a raised portion and a lowered portion with the attachment face of the movable contact being the reference are provided at the rear of and close to the movable contact on the movable plate, and a front end portion of the bimetal element is fitted in a gap formed by the raised portion and lowered portion.
In a preferred embodiment of the present invention, the raised portion and lowered portion are formed so that these portions have a substantially cranked shape in cross section, and the lower face of the raised portion is located at a position raised through a distance corresponding to the thickness of the movable plate from the upper face of the movable plate, so that the upper face of the front end portion of the bimetal element fitted in the gap substantially coincides with the contact attachment face of the movable plate.
In a preferred embodiment of the present invention, a protective cover is provided over the switching mechanism section, and a convex portion for pressing the upper face of a central portion of the bimetal element is formed on the lower face of the protective cover.
It is preferable that the projection height of the convex portion be set so that after an increase in curvature of the bimetal is started by a rise in ambient temperature, a pressing force is released before the ambient temperature rises to the reversing operation temperature of the bimetal element.
In a preferred embodiment of the present invention, a gap formed by the raised portion and lowered portion is provided in a central portion in the width direction of the movable plate.
In a preferred embodiment of the present invention, a gap formed by the raised portion and lowered portion is provided at both sides in the width direction of the movable plate.
In a preferred embodiment of the present invention, the fixed conductive plate is fixed integrally to a resin-made support surrounding the side periphery of the switching mechanism section, and the protective cover is formed of a metallic sheet and is disposed on the upper face of the support.
In a preferred embodiment of the present invention, protrusions are projectingly provided on the upper face of the support, and the protective cover is fixed to the support by fitting holes formed around the cover on the protrusions and then by crushing top portions of the protrusions.
In a preferred embodiment of the present invention, a resin-made frame is integrally formed around the protective cover, and the cover is fixed to the support by welding the frame onto the upper face of the support by ultrasonic welding.
It is preferable that the outside surface of the fixed conductive plate and/or the protective cover provided on the support be covered with an electrical insulating film with a small thickness.
Also, it is preferable that the electrical insulating film have a thickness not greater than 50 xcexcm.
The present invention provides a thermal protector having a switching mechanism section comprising a fixed conductive plate which has a first terminal for connection to an outside circuit and is provided with a fixed contact on the upper face thereof; an elastic movable plate located over the fixed conductive plate, which has a second terminal for connection to an outside circuit at the rear end thereof and is provided with a movable contact opposed to the fixed contact on the lower face of a front end portion thereof; and a bimetal element located over the movable plate, which operates the movable plate by a warping force when a preset temperature is exceeded, whereby the movable contact is separated from the fixed contact, wherein a raised portion directed to the rear side from the rear end of a front end portion of the movable plate including the movable contact is formed by bending a portion close to the rear end of the movable contact into a step form so that the front end portion is higher than a portion at the rear of the front end portion, and a front end portion of the bimetal element is fitted in a gap formed by the raised portion.
In a preferred embodiment of the present invention, the raised portion is formed so that the lower face thereof is located at a position raised through a distance corresponding to the thickness of the movable plate from the upper face of the movable plate, so that the upper face of the front end portion of the bimetal element fitted in the gap substantially coincides with the contact attachment face of the movable plate.
In a preferred embodiment of the present invention, a protective cover is provided over the switching mechanism section, and a convex portion for pressing the upper face of a central portion of the bimetal element is formed on the lower face of the protective cover.
In a preferred embodiment of the present invention, the projection height of the convex portion is set so that after an increase in curvature of the bimetal is started by a rise in ambient temperature, a pressing force is released before the ambient temperature rises to the reversing operation temperature of the bimetal element.
In a preferred embodiment of the present invention, a gap formed by the raised portion is provided in a central portion in the width direction of the movable plate.
In a preferred embodiment of the present invention, a gap formed by the raised portion is provided at both sides in the width direction of the movable plate.
The present invention provides a thermal protector having a switching mechanism section comprising a fixed conductive plate which has a first terminal for connection to an outside circuit and is provided with a fixed contact on the upper face thereof; an elastic movable plate located over the fixed conductive plate, which has a second terminal for connection to an outside circuit at the rear end thereof and is provided with a movable contact opposed to the fixed contact on the lower face of a front end portion thereof; and a bimetal element located over the movable plate, which operates the movable plate by a warping force when a preset temperature is exceeded, whereby the movable contact is separated from the fixed contact, wherein a portion close to the rear end of the movable contact is bent into a step form so that a front end portion of the movable plate including the movable contact is higher than a portion at the rear of the front end portion, and a hole is opened in a rising face formed by the bending operation so that a front end portion of the bimetal element is fitted in the hole.
The present invention having the construction as described above achieves the following effects.
1) A thin shape can be realized. In particular, the thickness can be made not greater than 1 mm, so that the thermal protector in accordance with the present invention can be suitably used for a battery pack for small-sized equipment such as a cellular phone.
2) In spite of the thin shape, both of OFF and ON switching operations can be performed normally by using the bimetal element. Specifically, a switching operation, in which the reversing operation of the bimetal element and the opening/closing operation of the contact are accomplished synchronously, can be performed.
3) Since the movable contact slides due to a wiping operation, the contacting properties of contacts are stabilized, resulting in improved reliability.
4) In spite of the thin shape, the switching operation is performed normally while a high contact pressure of the contacts is secured. Therefore, performance equivalent to that of a protector having a thickness two times and more the thickness of the protector in accordance with the present invention can be ensured.